This invention relates to opto-electronic encoders, and more particularly to transducers useful in determining position and/or velocity of one member with respect to another member.
In the past, opto-electronic encoders have been devised for determining position (angular or linear) in the form of binary coded tracks wherein the full extent of possible positions (i.e., the full extent of travel of one element with respect to another) is divided into two parts. Another track is divided into four parts, and so forth, the nut track being divided into 2.sup.n parts, with alternate parts of each track being opaque and the other parts transparent. A separate one of "n" photosensors is provided over each track to sense light transmitted from a source on the other side. The light detected at any given time by the group of n photosensors indicates in binary coded form position or extent of travel from a zero (reference) position with a degree of resolution that the length of the divided parts in the nth track bears to the total extent of possible travel. The static output of such a group of photosensors could be arranged in any other known binary code, such as the reflected binary or Gray code, by simply rearranging the separate parts of the tracks relative to each other.
The "n" track encoder offers a direct bit-parallel fully encoded output but accomplishes this through substantial numbers of photosensors and associated electronic circuits. In some applications such cost and complexity is not justified. It is often desirable to determine position by counting up cycles during motion in one direction and counting down cycles during motion in the opposite direction, starting with zero at a reference position. An advantage in determining position in that manner is that one track yields position data and if necessary velocity data. It is sometimes necessary to determine velocity as well as position.
A measure of velocity in the single track encoder is the rate at which the photosensor output of the position track makes excursions between maximum and minimum. Systems where velocity is determined from that rate by effectively counting cycles per unit time are not restricted however to single track arrangements. In the multi-track encoder previously described, the rate of change of code from the nth track can be used as a measure of velocity.
In general, the lowest cost and greatest reliability will be obtained with the least number of tracks and photosensors.
Regardless of the number of tracks or photosensors employed, it is necessary to assure in some manner or by some means that a relatively constant peak amplitude output signal is developed by the photosensor as the relative position of the moving element to the stationary element varies over the width of one opaque part and an adjacent transparent part. Since the photosensor output may be proportional to incident light, a constant amplitude may not be maintained without some means for controlling the light source intensity. This is particularly true in the case of a light emitting diode (LED) since the intensity of the light produced is highly dependent on applied current and device temperature as well as device age.
This need to provide some means of controlling the light source intensity has heretofore been recognized in various types of systems. Representative systems are disclosed in U.S. Pat. Nos. 3,775,617, and 3,809,895. In the first of these patents, a servo-indicator dedicates a second photosensor to continually monitor the intensity of light from an incandescent lamp and thereby provide a signal for controlling the intensity of the light source. In the second of these patents the output signals of a plurality of photosensors (employed in a system for measuring relative displacement of a scale) are combined linearly to provide a control signal the mean value of which is representative of the intensity of light radiated by a source, thus effectively monitoring the intensity of light from a source without dedicating a photosensor for that purpose. However, that technique is useful in only the particular arrangements disclosed of photosensors disposed along the line of relative scale motion, and phase displaced such that only one is receiving full radiation while all others receive only partial radiation. Still another displacement measuring system disclosed in U.S. Pat. No. 3,872,301 employs two apertures 180.degree. out of phase in a mask and separate photosensors. The outputs of the photosensors are differenced to obtain a position signal and added to provide a control signal to maintain the intensity of radiation constant. This system also has the advantage of not requiring a dedicated photosensor to monitor the intensity of the light source, but is uniquely dependent on the particular differencing technique for obtaining the position signal.
In many applications, it is desirable to continuously determine relative displacement (linear or angular) of a scale relative to an index position without an added photosensor being dedicated to monitoring of light source intensity, and with a minimum of photosensors. Using a scale having a transmission grating of, for example, 200 lines to the inch and a reticle with a section of the same grating as the scale, light passing to a photosensor through the reticle and scale from a source is modulated as the relative position of the scale over the reticle is changed. To determine position at any given time, cycles of the modulated output of the photosensor may be counted up and down for motion away from and toward an index position. To facilitate the determination of direction of motion, a second scale and reticle combination may be provided in phase quadrature with the first scale and reticle combination. The output signal of a second photosensor opposite the second reticle may then be compared with the output of the first photosensor for determining direction of motion. In either case, the rate of the cycles in the modulated outputs of the photosensors will provide speed information.
This arrangement of a transducer for position, speed and direction information utilizes only two photosensors. In addition to the two reticle photosensors, only one additional photosensor is required to determine an index position. For example, in the case of a linear scale, an index position at each end of the scale can be determined by providing a single bar at each end of the scale on a parallel transparent track. A single reticle slit positioned to illuminate a third photosensor will modulate the output of the third photosensor to provide a single pulse at each index position of the scale.
The usefulness of this indexed scale-reticle arrangement will depend upon having a relatively constant peak amplitude of the output signals, that are developed by the photosensors. Output signal peak amplitude must be predictable within certain limits as this signal is processed by electronic circuitry which cannot accommodate an extreme range of amplitude variation, or, for some applications, the position signal is used directly as the position feedback in a closed-loop servo-system, and the amplitude must be controlled in order that the gain of the servo-system can be predicted. Without this degree of predictability of gain within the servo-system, the servo-system may not achieve the desired sensitivity or position accuracy or it may become unstable.
The limit of predictability that can be achieved will depend upon the extent to which the light source intensity can be regulated. A control loop would provide optimum regulation, but the problem is to provide an optimum control loop without additional photosensors. A very tight control loop would, of course, include the photosensor that provides the position signal; otherwise, variations in the light detecting characteristics of the photosensor will cause variations in the position signal. However, the variations in the characteristics of some photosensors with temperature, age and other factors is at least an order of magnitude smaller than variations in the radiating characteristics of light sources and the variations in effective transmissibility of the optical path. This is particularly true in the case of light emitting diodes. Consequently, it is not as imperative that the actual position photosensor be included in a control loop to regulate the light source intensity. However, it is important that the means of detecting the intensity of the light source for use in the control loop be subjected to the same environment as the position photosensor, and that it experience light intensity variations that are of the same nature as that experienced by the position photosensor which result from variations of the effective transmissibility of the optical path.
In practical opto-electronic position transducers, the amount of light impinging on the photosensors departs from that of the ideal case. This is due to a variety of causes, and this variation from the ideal case may be treated as a variation in the effective transmissibility of the optical path of the transducer. A portion of these variations from the ideal is experienced in different amounts between different units of a given design. Further, within any given unit, variations will occur due to the nature of the practical elements employed in the optical path. In addition, degradation of the optical path may occur during actual use of the transducer due either to the environment in which the transducer is used or due to aging of the elements in the transducer.
By way of example, some of the causes of variation in the effective transmissibility are: changes in the optical density of any lenses and/or the scale and reticle of the system due to condensation of atmospheric contaminants; variations in the effective optical density of the scale, over the length of the scale, due to imperfections in the manufacture of the scale; and scattering of light from the light source due to dust and other particulate matter in the atmosphere in the optical path of the light. Regardless of whether the change in the effective transmissibility of the optical path is due to the use of the transducer in a given environment or caused by the nature of manufacture of the transducer elements, it is highly desirable to be able to regulate the light source intensity to provide compensation for these effects, in addition to compensating for any changes in the ability of the light source to maintain a constant light intensity. Further, if by design the photosensor that is within the control loop is subjected to the same range of temperature and other environmental effects as the photosensor which is used for producing the basic position signal, a near total compensation can be achieved.